The sites of fatigue of human respiratory muscles have been compared with those observed in various limb muscles of different function and fiber type composition. Fatigue of the diaphragm was induced in normal subjects by either: a) breathing against an inspiratory resistance; or b) performing expulsive abdominal maneuvers, which minimize the use of intercostal and accessory muscles. With both protocols, the subjects were required to generate square-wave increases in transdiaphragmatic pressure (Pdi) of the required value (%Pdi max) using a 0.6 duty cycle at 12 breaths/min until the target Pdi could no longer be reached (Tlim). In most cases, diaphragm shortening was minimized by abdominal binding, so that the contractions were quasi-isometric. Similar patterns of intermittent, submaximal limb muscle contractions were performed at comparable relative forces (%MVC). For each muscle type, the responses to muscle stimulation interposed between voluntary contractions were measured for changes in muscle contractile strength; those superimposed on the target force, for changes in the relative degree of voluntary CNS activation. The evoked muscle M-waves were monitored for potential impairment of neuromuscular transmission. For stimulation of the diaphragm, supramaximal shocks were delivered bilaterally to the phrenic nerves. The results for limb muscles show that the loss of force during fatigue can be accounted for by peripheral contractile failure, and that the CNS can still cause full muscle activation. For the diaphragm, similar maximal CNS muscle activation was demonstrated prior to fatigue, but could not be achieved thereafter (superimposed twitches were still elicited during maximal efforts, and the estimated muscle strength always exceeded that which could be generated voluntarily). No evidence was seen for failure of transmission in either system. Thus, CNS inhibition appears to be an important factor in fatigue of respiratory muscles. We have shown that during fatigue of limb muscles, loss of force is accompanied by a decline in the maximum motor neuron firing frequency as the muscle contractile speed becomes slower. Since this slowing reduces the tetanic fusion frequency, the decline in discharge rates does not limit CNS muscle activation. The parallel between the changes in contractile speed and motor neuron firing rates suggests a regulating mechanism. We propose to explore whether this regulation resides within the CNS, or is due to a reflex from the fatigued muscle.